Two different construction designs are commonly used for reciprocating piston air compressors. According to one design, a generally cylindrical shaped piston is constrained to slide in a cylinder. A connecting rod is secured at one end to the piston with a wrist pin to permit rotation between the piston and the connecting rod. An opposite end of the connecting rod is secured to be rotated by a crank pin on a motor driven crank shaft or on an eccentric. As the crank pin is rotated, the connecting rod converts the rotary motion to reciprocate the piston. The piston is provided with one or more piston rings to form a sliding seal between the piston and the wall of the cylinder to prevent gas leakage from a compression chamber formed by the cylinder and piston. In order to minimize friction and wear, the connecting rod connections and the cylinder walls and piston rings must be constantly lubricated during operation. Consequently, oil is provided to lubricate these surfaces during operation of the compressor. One disadvantage with an oil lubricated air compressor is that some oil may pass between the cylinder walls and the sliding piston ring seals into the compression chamber. Any oil which enters the compression chamber will mix with the compressed air. For some applications, it is undesirable to have any oil mixed with the air. For example, when using compressed air to operate a paint spray gun, any oil in the air may adversely affect the quality of the applied paint. Also, oil in the compressed air may be undesirable when the compressed air is used with a dusting gun.
When higher air pressures are needed, air compressors frequently are provided with two stages of compression, i.e., with two cylinders. A first stage compresses the air to an intermediate pressure and a second stage increases the intermediate pressure air to a desired higher level. Since the air delivered from the first stage to the second stage is partially compressed and has a smaller volume than the air initially delivered to the first stage, the second stage will have a smaller displacement than the first stage. This generally has been accomplished by making the diameter of the second stage piston smaller than the piston diameter for the first stage. Normally, the crank shaft provides the same stroke length for the two pistons. In U.S. Pat. No. 1,067,770 to Spohrer, it was recognized that when the second stage piston was made significantly smaller than the first stage piston, the bearing size for the connecting rod bearing surfaces at the piston also had to be made significantly smaller. Since the bearings in the second stage are subjected to higher pressures in than in the first stage, the smaller bearing size could result in excessive wear and premature bearing failure. According to this patent, the second stage was provided with a shorter piston stroke than the first stage and the diameter of the second stage piston was increased to retain the desired displacement. Although the second stage piston remained smaller than the first stage piston, the increased diameter of the second stage piston permitted the use of a larger bearing between the connecting rod and the second stage piston to prolong the bearing life.
A second design for reciprocating piston air compressors does not require oil lubrication. In an oil free compressor, the piston consists of a connecting rod and a piston head formed as a single integral unit so that there is no rotation between the connecting rod and the piston head. A free end on the connecting rod is connected to be rotated by a crank pin on a motor driven crank shaft or other eccentric. The piston head has a smaller diameter than a cylinder in which it is reciprocated to permit the piston head to rock or wobble in the cylinder, since the connecting rod and piston head are integral. A flexible cup shaped seal is secured to the piston head to seal with the walls of the cylinder as the piston head is reciprocated and wobbles. Oil free air compressors have the advantage over oil lubricated air compressors in that oil will not leak past the seal where it can mix with the compressed air. However, they have a disadvantage in that the cup shaped seal has a more limited operating life than oil lubricated piston rings. The seal life is determined in part by the air pressure applied to the seal and by the velocity and the distance that the seal travels in each stroke. As the pressure increases, the seal is pressed tighter against the walls of the cylinder. Consequently, the seal is subjected to greater wear at higher compression pressures.
Two stage oil free air compressors have been attempted in the past. These have been constructed with pistons of the type having a connecting rod connected to the piston with a wrist pin. It is believed that these compressors were operated at a relatively slow speed in order to extend the life of the piston ring seals. Although single stage wobble piston oil free air compressors have been highly successful, two stage wobble piston oil free air compressors have not been made due to excessive wear on the second stage seal. The second stage seal would require replacement long before replacement is needed for the first stage seal. Consequently, higher pressure reciprocating piston air compressors have not been of the oil free wobble piston type.